Method of electrotinning continuous steel strip



y 1954 R. E. HOWELL 2,677,652

METHOD OF ELECTROTINNING con'rmuous STEEL STRIP Filed July 2, 1949 I00 I L 80 I j CURRENT DENSITY (AMPERES PER SQ. FOOT) PLATING PASS NUMBER INVENTOR. ROY E. HOWELL HIS ATTORNEY Patented May 4, 1954 METHOD OF ELECTROTINNING CONTINU- OUS STEEL STRIP Roy E. Howell, Pittsburgh, Pa., assignoi' to United States Steel Corporation, a corporation of New Jersey Application July 2, 1949, Serial No. 102,778

2 Claims. 1

This invention relates to a method of electrotinning steel strip and in particular to a method of operation of the acid plating baths used in continuous electrotin'ning processes. The present invention contemplates the elimination in such a process of the production of the defect known as wood grain in the finished product.

Tin plate carrying coatings up to 1.0 lb. per base box is commonly made by passing a strip of low carbon steel at substantial speeds through a series of processing steps wherein the strip is electroplated with tin and is heated through the melting point of tin to brighten the coating. Acid type plating baths are frequently employed for the coating ste since they permit the use of higher current densities; consequently, the use of shorter plating tanks. To further conserve space, the plating is usually conducted in substantially vertical passes, the strip being passed over an entry conductor roll, downwardly through the plating solution around a sink roll and upwardly to a second conductor roll, hence downwardly, etc. The submerged portion of the strip between a negative conductor roll and a sink roll is termed a plating pass. Several of such passes are arranged in sequence so as to provide the necessary length of plating bath. The weight of coating deposited is controlled by regulating the amount of current applied to the strip during its passage through the plating tank.

In spite of adherence to the best known operating practice a defect known to the trade as wood grain is often encountered in the acid tin strip plating processes. This defect is objectionable from an appearance standpoint and makes the plate somewhat more difficult to solder. The defect closely resembles the longitudinal grain in wood and generally runs at right angles to the direction of rolling and plating. The pattern is readily discernible on the material after the melting operation.

The exact cause of wood grain is not known. It is most often encountered when plating tin coatings of less than 0.5 lb. per base box. Both the frequency of occurrence and the intensity of the grain-like pattern decrease in the heavier coatings. The defect appears to be peculiar to the use of acid tin plating baths, i. e., it is not known to occur when alkaline tin plating baths are used. 7

The method of heating used in the brightening step also appears to play a part in the appearance of the defect, e. g, the defect is a relatively common occurrence on commercial lines using resistance melting as the 'means for brightening, but when the defect is encountered on such a line and samples are taken immediately after plating, the characteristic pattern of wood grain is not produced when the coating is melted in a hot oil bath or over a gas flame. The melted coatings on the samples, howeven'are dull in areas irregular in shape and extent.

Regardless of the theoretical cause or causes of the defect, I have found that wood graining can be avoided by conducting commercial plating operations under the following conditions: The plating current must be proportioned to the individual passes so that (l) the initial plating is conducted at very low current density, and (2), the final plating must be conducted at a current density within what is known as the preferred operating range of the particular solution being used. The total current applied in all the passes, of course, must be sufficient to produce the desired weight of coating. The invention is more readily understood from the following detailed description when read in conjunction with the attached drawing which is a chart illustrating the current density practices of the present invention.

Referring specifically to the attached chart, the ordinate is current density in ampers per square foot, and the plating passes of an eightpass tank arrangement are indicated by the numbers along the abscissa, number 1 indicating the entry pass; number 8, the final (exit) pass. The points plotted are the current densities applied in the individual passes. The points connected by the lines A, B, C, and D represent four varieties of practice all in accordance with the concepts of the present invention. All will produce plate free of wood grain and equivalent in other respects to plate made under the best known prior practice. The plotted data refer specifically to quarter-pound per base box tin coatings produced at 5'70 feet per minute using a phenolsulphonic acid type tin plating bath and a plating tank having eight plating passes, each pass providing about five feet of effective length of plating. The horizontal lines :1: and 1 define the upper and lower limits, respectively, of the heretofore preferred operating range of the plating bath. In the practice depicted in line A the current density is maintained at about 15 amperes per square foot in the first pass, at 35 amperes per square foot in pass No. 2, at amperes per square foot in pass No. 3, at amperes per square foot in pass No. 4, at amperes per square foot in pass No. 5, at 1-35 amperes per square foot in pass No. 6, at amperes per square foot in pass No. 7 and at 270 amperes per square foot in pass No. 8. The latter value is well within the preferred operating range of the solution.

In the practice depicted by line B the current density is maintained at 10 amperes per square foot'in the first pass and is increased to 20, '50, 75, 1 10, 150,200 and 255 amperes per square foot in the succeeding passes. Practice B differs from A mainly in that the current density in pass 7 as well as in pass 8 is within the normally preferred operating range.

In practice C a current density of about 10 amperes per square foot is maintained in the first pass, in the second, 35 in the third, 60 in the fourth, 165 in the sixth an 240 amperes per square foot in the seventh and eighth passes. Practice C is therefore similar to practice 13 except that a constant current density within the preferred operating range is maintained in passes 7 and 8.

In practice D the current density is maintained at about 10 amperes per square foot in both the first and second passes; at 95 amperes per square foot in the third, fourth, and fifth passes; and at 190 amperes per square foot in the sixth, seventh, and eighth.

The four practices merely illustrate operations under the present invention. It is evident that numerous variations are possible since to produce coatings free of wood grain and satisfactory in all other respects, it is essential only that the initial plating be conducted at a current density below amperes per square foot, preferably the current density is maintained as near zero as is practically possible, and that the final deposit be applied at a current density which is known to produce coatings of good adherence, uniformity, meltability, etc. It is characteristic of the acid tin plating baths that good adherence, uniformity, meitability, etc. are only obtained when the coating is deposited within a limited range of current densities. his range varies for different acid plating baths. In a given type of acid bath, it varies with the temperature, the tin and acid contents, and the addition agent used. This limited range is known as the preferred operating current density range. Heretofore, it was believed necessary to conduct the entire plating within this range. I have found, however, that only the final plating need be applied within this range to impart these properties to the coating as a whole. Plating in several of the passes toward the exit end of the plating tank may be conducted within this range if desired as for example in practice D. It is essential however that at least the plating in the last pass must be conducted within the preferred operating range. If the entire coating were deposited at the low current density used in the first pass the product while free of wood grain would be otherwise unacceptable.

The current densities in the intermediate passes are not critical insofar as the properties of the coating are concerned. The weight of coating deposited however is a function of the total plating current and the time of plating. Sufiicient current therefore must be applied to the strip during its passage through the total of the plating passes to provide the desired weight of coating. This can easily be accomplished by regulating the current density in the intermediate passes. The current density in these passes can be either progressively increased in each succeeding pass, as for example in practices A and B, or increased in groups of passes as indicated by practice D.

The present invention can be practiced by providing separate generators of suitable capacity for supplying current to the individual passes or in operations of the type D by providing separate generators for groups of passes. Except for the critical conditions of plating current set forth above electrotinning operations are conducted in the usual manner, that is, a strip of annealed low carbon steel is passed through equipment arranged in sequence wherein it is cleaned or pickled, rinsed, plated, rinsed and dried, subjected to sufficient heat to melt the coating deposited thereon, quenched, dried and oiled, after which it may be coiled or cut up into sheets of desired length. These operations and the equipment for carrying them out are well known and have not been described.

While I have shown and described certain specific embodiments of my invention, it will be apparent that other adaptations and modifications may be made without departing from the scope of the following claims.

I claim:

1. The method of continuously electrotinning steel strip in acid tin plating baths to prevent wood grain and like defects comprising passing said strip in a plurality of plating passes through an acid tin plating bath having a preferred opcrating range of current density, each of said passes including a negative contact roll for establishing the strip as cathode, a sink roll disposed in the bath and anodes disposed on either side of the strip in the bath between said contact roll and said sink roll, applying sufiicient current to said strip during the passage through the total of said passes to deposit a desired weight of coating thereon, the current density in at least the first of said passes being maintained substantially below the preferred operating range of the bath when used for continuously plating moving strip and at least below 20 amperes per square foot, the current density in at least the last of said passes being maintained at a value within said preferred operating range of said bath.

2. The method of continuously electrotinning steel strip in acid tin plating baths to prevent Wood grainaand like defects comprising passing said strip in a plurality of plating passes through an acid tin plating bath having a preferred operating range of current density, each of said passes including a negative contact roll for establishing the strip as cathode, a sink roll disposed in the bath and anodes disposed on either side of the strip in the bath between said contact roll and said sink roll, applying sufficient current to said strip during the passage through the total of said passes to deposit a desired weight of coating thereon, the current density in at least the first of said passes being maintained substantially below the preferred operating range of the bath when used for continuously plating moving strip and at least below 20 amperes per square foot, increasing the current density in each succeeding pass, the current density in at least the last of said passes being maintained within the preferred operating range of said bath.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,313,371 Stack Mar. 9, 194-3 2 370,986 Nachtman Mar. 6, 1945 2,382,018 Martin Aug. 14, 1945 2,450,795 Harris Oct. 5, 1948 2,473,918 Stoltz et a1 June 21, 1949 OTHER REFERENCES Modern Electroplating, special edition of The Electrochemical Society, 1942, page 321. 

1. THE METHOD OF CONTINUOUSLY ELECTROTINNING STEEL STRIP IN ACID TIN PLATING BATHS TO PREVENT WOOD GRAIN AND LIKE DEFECTS COMPRISING PASSING SAID STRIP IN A PLURALITY OF PLATING PASSES THROUGH AN ACID TIN PLATING BATH HAVING A PREFERRED OPERATING RANGE OF CURRENT DENSITY, EACH OF SAID PASSES INCLUDING A NEGATIVE CONTACT ROLL FOR ESTABLISHING THE STRIP AS CATHODE, A SINK ROLL DISPOSED IN THE BATH AND ANODES DIPOSED ON EITHER SIDE OF THE STRIP IN THE BATH BETWEEN SAID CONTACT ROLL SAND SAID SINK ROLL, APPYLING SUFFICIENT CURRENT TO SAID STRIP DURING THE PASSAGE THROUGH THE TOTAL OF SAID PASSES TO DEPOSIT A DESIRED WEIGHT OF COATING THEREON, THE CURRENT DENSITY IN AT LEAST THE FIRST OF SAID PASSES BEING MAINTAINED SUBSTANTIALLY BELOW THE PREFERRED OPERATING RANGE OF THE BATH WHEN USED FOR CONTINUOUSLY PLATING MOVING STRIP AND AT LEAST BELOW 20 AMPERES PER SQUARE FOOT, THE CURRENT DENSITY IN AT LEAST THE LAST OF SAID PASSES BEING MAINTAINED AT A VALUE WITHIN SAID PREFERRED OPERATING RANGE OF SAID BATH. 